Prevention of white rust on galvanized sheets



T. SENDZIMIR 3,383,189

PREVENTION OF WHITE RUST ON GALVANIZED SHEETS May 14, 1968 Filed April 16, 1964 mvawron TADEUSZ Seuozwm United States Patent 3,383,189 PREVENTION OF WHITE RUST 0N GALVANIZED SHEETS Tadeusz Sendzimir, T. Sendzimir, Inc., Waterbury, Conn. 06720 Filed Apr. 16, 1964, Ser. No. 360,453 3 Claims. (Cl. 29-1965) In my Patent 2,110,893 I have disclosed a process of producing zinc coatings on steel strips characterized by a superior adherence due to the elimination of certain brittle iron-zinc alloys. Only one alloy is formed in a very thin layer, and that one is not brittle.

I obtained these results by slightly altering the surface layer of the basic steel strip so as to convert it into almost pure iron. But another contributing factor towards elimination of the brittle iron-Zinc alloys is an addition of a certain small percentage of aluminum to the Zinc coating bath. It sufiices that 0.2% of aluminum be dissolved in the zinc to obtain this result.

The addition of aluminum has, however, one drawback and, namely, that sheets and strips so coated with zinc, have a tendency to develop a so-called white rust which is a product of corrosion of the zinc coating, especially if the sheets are piled one upon another in a warehouse and if there is a certain amount of moisture present.

Certain means have been tried to prevent th appearance of such white rust as for instance coating such sheets with an inert fat or paraffin and also chemical means such as a diluted bath of chromic-oxide, but the effectiveness of such means is small.

Appicant has developed a process which radically solves this problem and which essentially consists in first obtaining an adherent zinc coating with only a very thin alloy formation, then immediately and before the molten coating has solidified, stripping the same and leaving substantially only the thin zinc-iron alloy coating and immersing the strip again into a molten zinc bath which contains no aluminum additions.

It has been found that if the operation is fast and certain temperature and other conditions are fulfilled, the originally formed thin layer of the iron-zinc alloy is not attacked or substantially altered by the subsequent short immersion in an aluminum-free zinc bath and the resulting fullweight coating is free of aluhinum if none a been added to the second bath.

The folowing figures explain my process better:

FIGURE 1 represents a schematical cross section of the coating baths.

FIGURE 2 represents a schematic cross section of another embodiment of my process.

FIGURE 3 represents an arrangement similar to that on FIGURE 1, but with an extended lead bath and a different wiping arrangement in the aluminum-free Zinc bath.

FIGURE 4 shows a detail of the wiping arrangement in FIGURE 3, in section along A-A.

Steel strip 1 is shown here as emerging from the pretreatment apparatus not shown and is contained within an air-tight hood 9 filled with a non-oxidizing atmosphere 2 to protect its surface from oxidation. It has a temperature about 50 higher than the temperature of the molten zinc bath 3.

The zinc bath 3 is shown here floating on top of a lead bath 4 and is laterally confined by the bottomless container 7.

Lead bath 4 is, itself, contained in a pot 6 which is preferably heat insulated and heated by suitable means not shown.

Said zinc bath 3 does contain a small percentage of aluminum as above indicated.

Another zinc bath which may contain other alloying ice elements as is known in the art but to which no aluminum has been added, is also floating on top of the lead bath 4 and is laterally confined by the bottomless container 8.

In its path through the baths, strip 1 receives its first zinc-aluminum coating when first submerged in bath 3 which coating is then scraped off by a suitable scraper apparatus 11 and it floats upwards and joins again the bath 3.

The non-brittle zinc-iron aTloy which is formed under those circumstances is not liquid at that temperature and is not scraped off. its thickness is usually less than 10% of the original coating.

Strip 1 is then led around the pulley 10 and upwards through scraper 12 and through the aluminum-free zinc bath 5 Where it receives its full-Weight zinc coating which is controlled as to thickness by conventional methods such as the exit roils 13 from which it exits vertically into the atmosphere where it is cooled and solidifies after which it is led over pu'ley 14 and other pulleys not shown for comp ete cooling and other operations.

While under ordinary circumstances, a properly insulated lead bath 4 will sufiice to keep the two Zinc baths 3 and 5 at their correct temperatures and will not require a regular supply of outside heat to maintain such temperatures, there being enough heat supplied by the excess heat of the entering strip 1, precautions should be taken to insure that the a'uminum-free bath 5 is preferably not hotter than the zinc aluminum bath 3. For best operation, this temperature should be kept as low as is consistent with obtaining a good aspect of the coating.

FIGURE 2 represents another embodiment of the invention such as I prefer to use on high-speed lines where there might be a risk of dragging a small quantity of lead along with the zinc onto the finished strip 1 which has been treated by pre-oxidizing, then reducing at high temperature and finally cooling down to a temperature about C. higher than the temperature of the zinc coating bath as in FIGURE 1, and while still under protection of the non-oxidizing atmosphere within hood 9, is first immersed into the aluminum-containing zinc bath 3 (all suhstanttially as disclosed in said Patent 2,110,893), contatined in suitably heated pot 6 and is led around bottom pulley 10 and up substantially vertically through the scraper 11 which could be disposed just above the surface of zinc bath 3 and which wipes off the fluid zincplus-a uminum coating which has been formed on immersion of said strip 1 in the bath 3 under optimum conditions of zinc composition and temperature, to produce such thin tightly adherent and not brittle coating on the base steel strip surface.

The operation of stripping the part of that coating which is not alloyed with iron, is obtained under conditions most favorable for such stripping and, namely, at a temperature of slightly less than the temperature of the molten zinc bath 3 and while the non-alloyed portion of the coating is quite liquid and. therefore readily strippable whereas the very thin zinc iron alloy coating is solid or almost solid and resists stripping.

It seems not very important which type of a wiper is used and they can be either of the non-wetting or of the wetting type but I have found that a steel wiper of medium hardness and having a radius of about 0.020 on the contacting blade, gives good results. As is known in the art, such wipers should be pressed into the steel strip so as to cause a slight deflection of it and the preferred. embodiment is that, when two such wipers are used on one side against one Wiper on the other, so that any slight buckle or wavy edges, as often happen on steel strips, will not badly disturb the stripping action, as they would if one wiper were placed directly opposite another wiper contacting the opposite face of the strip.

Strip 1 after passing through wiper 11 is led around the pulley 21 and led downwards again into the second zinc bath contained. in pot 16, also suitably insulated and heated.

On its passage between the two baths 3 and 5, strip 1 is preferably protected by a non-oxidizing atmosphere, contained in hood 23 which may have a lid 25 to permit occasionally access to the wiper 11. Such lid must of course be gas-tight which can be easily done by providing troughs 24 all around the upper edge of hood 23 filled with liquid metal e.g., lead 26, into which the down-turn lip of cover 25 can dip.

Protection against oxidation of the zinc-iron alloy coating on strip 1, on its trip from bath 3 to bath 5, is not essential but it is highly desirable since it produces a cleaner bath. If the temperature of strip 1 does not drop too low before its entry into bath 5, a certain amount of oxide on the surface of the zinc will not prevent formation of a sound secondary coating of the aluminum-free zinc.

After immersion in bath 5, strip 1 is led around bottom pulley 22 and substantially vertically up, through any type of coating equalizing apparatus such as conventional exit rolls 13 after which it is permitted to cool down below the crystallizing temperature and then led over pulley 14 into subsequent apparatus for complete cooling.

The temperature and other conditions as well as the length of travel within the finishing zinc bath 5 must be at optimum values for the production of a coating of the necessary weight, aspect and other characteristics. Such temperature should preferably be as low as is consistent with obtaining good characteristics of the coating as otherwise there might be a danger of damage to the controlled iron-zinc alloy layer which has been produced in bath 3.

In all cases where it is desirable to complete the primary foundation coating by the finishing zinc coating, without letting the strip leave the metal bath, it is possible to obtain that result and at the same time have high operating speed by increasing the dwell of the strip in the lead bath 4, FIGURE 1. For this purpose, said lead bath could be lengthened. as shown in FIGURE 3 and two bot-tom pulleys and 10 can be provided for correct leading of the strip.

While bath 4 has been designated as a lead bath, it must be understood that any metal or alloy whose specific gravity is higher than the gravity of zinc and that does not readily alloy with either iron or zinc, can be used for this purpose. Lead fulfills that requirement fairly well.

It must be understood, however, that whenever it is desirable to lead the strip 1 from the lead bath upwards through the zinc bath at a fast rate of speed, there is always a danger of mechanically dragging globules of lead into the secondary zinc bath and then up with the strip so that they solidify as a part of the zinc coating. As long as the quantity is negligible, this in itself is not very bad but I prefer to guard against it by using a rotating instead of a stationary wiping device, 12', FIGURE 3. Such a device may consist of two gear wheels 31 with smoothly rounded teeth, synchronized with each other and rotating in the direction opposite to the movement of strip, in such a way as to flex the strip very slightly each time when a tooth of one gear wheel faces a gap between two teeth on the other. Such flexing must be very small and only enough to push back any entrapped lead but not enough to disturb the original controlled iron-zinc alloy layer. I prefer to make the gears out of a non-wetting material, e.g., a suitable ceramic material and to make the teeth herringbone and not straight, so directed as to throw the lead particles downwards and sideways of the strip, although straight teeth also give good results.

Since such rotary wipers Will produce some churning of the finishing zinc bath in which they operate, I prefer to make said zinc bath shallow but wider than would ordinarily be required so as to have a greater mass of zinc in it in order to partly amortize such churning action.

I prefer to drive said rotary wiper from below, through the lead bath as shown in FIGURE 4 in a longitudinal view.

A part of the lead bath 4 is shown contained in pot 6 with the finishing zinc bath 5 contained in the bottomless hood 8 and floating on said lead bath 4. Within said. lead bath is shown a pair of round tooth herringbone or straight pinions 31 located in bearings 32 and driven over a pair of spur gears 33 from a shaft 36 totally submerged in lead and from there over a pair of bevel gears 34, from a gear m0- tor 35. Means for adjusting the relative position of the two pinions are known in the art and are not shown here.

If desired, these driven wiper pinions can be conveniently used for the drive of exit rolls 13 whose conventional drive from the zinc bath is subject to a lot of wear and tear because of the erosive action of zinc, whereas here, the exit rolls which have to rotate in the same direction as the strip but not necessarily at the same speed as the strip, can be conveniently driven by friction, preferably from toothless end portions 37 of said wiper rolls. Since half of these rolls is situated above the level of the zinc bath 5, their weight is sufficient to make such a drive operative.

The lengthening of that bath as shown in FIGURE 3 permits to separate better physically the two baths of zinc, 3 and 5 so as to obtain in each one the temperature conditions optimal for each operation and more particularly, to have the temperature of the finishing zinc bath 5, at the exit, low enough for best results.

Having thus described my invention, what I claim is:

1. The method of applying tightly adherent hot dip zinc coatings onto a continuously running length of ferrous material which comprises immersing said material in a molten zinc bath containing sufficient aluminum up to approximately 0.2% to form a controlled very thin pliable zinc-iron alloy layer exhibiting non-brittle characteristics, subsequently wiping off all the molten zinc and aluminum which is not alloyed with iron, and immediately afterwards passing said material with its newly-formed controlled zinc-iron alloy layer through a molten zinc bath substantially free of any aluminum.

2. The method set forth in claim 1 wherein the lastmentioned molten zinc bath is at a slightly lower temperature than that of the first zinc-aluminum alloy bath.

3. A coated length of ferrous material produced in accordance with the method set forth in claim 1.

References Cited UNITED STATES PATENTS 2,063,721 12/1936 Bradley 117114 X 2,258,327 10/1941 Kramer 11771 X 2,428,523 10/1947 Marshall 117114 X 2,914,419 11/1959 Oganowski 117-114 X 2,950,215 8/1960 Slater et a1. 117114 2,952,568 9/1960 Diehl et a1. 117114 X 3,078,555 2/1963 McFarland 117-71 X 3,177,053 4/1965 Lusa 29-196.5 3,190,768 6/1965 Wright 117114 X 2,111,826 3/1938 Waltman et a l. 1171l4 FOREIGN PATENTS 876,032 8/1961 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

J. R. BATTEN, JR., Assistant Examiner. 

1. THE METHOD OF APPLYING TIGHTLY ADHERENT HOT DIP ZINC COATINGS ONTO A CONTINUOUSLY RUNNING LENGTH OF FERROUS MATERIAL WHICH COMPRISES IMMERSING SAID MATERIAL IN A MOLTEN ZINC BATH CONTAINING SUFFICIENT ALUMINUM UP TO APPROXIMATELY 0.2%, TO FORM A CONTROLLED VERY THIN PLIABLE ZINC-IRON ALLOY LAYER EXHIBITING NON-BRITTLE CHARACTERISTICS, SUBSEQUENTLY WIPING OFF ALL THE MOLTEN ZINC AND ALLUMINUM WHICH IS NOT ALLOYED WITH IRON, AND IMMEDIATELY AFTERWARDS PASSING SAID MATERIAL WITH ITS NEWLY-FORMED CONTROLLED ZINC-IRON ALLOY LAYER THROUGH A MOLTEN ZINC BATH SUBSTANTIALLY FREE OF ANY ALUMINUM.
 3. A COATED LENGTH OF FERROUS MATERIAL PRODUCED IN ACCORDANCE WITH THE METHOD SET FORTH IN CLAIM
 1. 